The central goal of the Transgenic and Knockout (TKO) Core at the Mayo Clinic Cancer Center is to generate genetically modified mice for MCCC investigators investigating the normal and malignant functions of genes implicated in human cancers. Dr. Jan van Deursen, who has 25 years of experience in generating transgenic and gene-targeted mouse strains, directs the TKO Core. Two experienced technicians. Dr. Wei Zhou and Ms. Ming Li, execute projects on a first-come-first-serve basis. In practicality this means that most projects are initiated within one to two weeks after a request for service. Our main services include: pronuclear microinjection of DNA into fertilized eggs for the production of transgenic mice, transfection of DNA into ES cells for the generation of gene-targeted ES cells or other types of recombinant ES cell clones, injection of ES cell clones into blastocysts for the generation of chimeric mice, sperm cryopreservation, and recovery of cryopreserved mouse strains. These services are all-inclusive allowing investigators with little or no experience in generating transgenics or knockouts to implement genetically altered mice into their research projects. Success rates of all our services are high, with nearly all transgenic projects generating at least 2-3 independent founders and 85-90% of projects involving gene-targeted ES cells or other types of recombinant ES cell clones yielding the desired mutant strains. Service fees of the TKO Core are lower than external users fees for similar services at academic institutions in the US, and much lower than fees of for profit companies. The user base of the TKO Core, which has traditionally been very broad, has expanded dramatically over the current funding period, with 34 MCCC members from eight different programs using the TKO Core. The broadening of the user base is due to implementation of popular new services such as strain cryopreservation and rederivation. 32 of the 34 MCCC members using the TKO Core are extramurally funded. Importantly, a high proportion of successful extramural applications by MCCC members included genetically modified animals generated by the TKO Core.